OBJECTIVES: 1. Purified, angiotensinase-free substrate has been prepared from the serum of human, hog and nephrectomized dogs. This will be employed to carry out microassays of renin and antirenin from the serum and tissues of various species. 2. Continuation of previous attempts to localize sites of formation, storage and inactivation of antirenin. 3. Investigation of an angiotensin-producing factor which appeared in the serum of a dog during immunization with hog renin and production of antirenin. 4. Investigation (Isolation and Identification) of pseudorenin and of an inhibitor of pseudorenin present in normal human serum.